1. Field of the Invention
This invention relates to a method of selectively extracting low molecular weight solvents to concentrate higher molecular weight solutes in solutions therein by the use of temperature-sensitive cross-linked polymer gels. Such gels absorb low molecular weight solvents, such as water, but not high molecular weight solutes, such as proteins. The high molecular weight solute is recovered as a concentrated solution. In my aforesaid copending application Ser. No. 526,275, now U.S. Pat. No. 4,555,344. methods are disclosed and claimed by which the low molecular weight solvents are recovered by a change in pH of the gel or by a change in the composition of the surrounding liquid. According to the present invention, the low molecular weight solvents are recovered by slightly warming the gels. The warming causes a huge change in the amount of solvent absorbed by the gel, a shrinkage occurring because the gel is near a critical point, the temperature at which the gel is capable of undergoing a phase change.
The invention is important because separation processes are a key aspect of the chemical industry. In the past, these processes were dominated by distillation, reflecting the key role played by petroleum. Other important separations include gas scrubbing, liquid-liquid extraction, crystallization and filtration. However, there are an emerging group of separation problems for which current technology is expensive and difficult. These problems center around dilute solutions of organic or biological materials. Examples include the removal of water from dilute solutions like cheese whey, fermentation beers, and protein solutions produced by genetically-energineered microorganisms.
2. The Prior Art
Apart from the invention disclosed in my aforesaid copending application, the basic idea of using gels near a critical point as size-selective extraction solvents which can be regenerated by small temperature changes seems to be new and no relevant prior art is known. However, the use of gels for separations is not new, and phase transitions in gels have been previously reported.
Separations using gels are usually based on gel permeation chromatography (GPC). The basic apparatus used in this method consists of a packed bed of gel spheres of the same size and swollen with solvent to a constant extent. Additional solvent flows steadily through this bed. At time zero, a pulse of solution containing several high molecular weight solutes is injected at the top of the bed. As the pulse is swept down the column, different solutes are retained by the gel to different degrees. Basically, small solutes which can diffuse quickly into the gel are retarded the most, and large solutes which are excluded from the gel are swept along fastest. Thus the largest solutes are eluted most quickly, and the smallest solutes come out of the column last. The differential retention by any single gel sphere is very slight, but the total retention for all the spheres in the bed can effectively separate the solutes.
In the GPC separation, the separation is of very small amounts of similar high molecular weight solutes in a packed bed of gel swollen to a constant extent. Changes in swelling ruin the separation. According to the present invention, the separation is of potentially large amounts of one solute and a solvent using a gel whose swelling is deliberately altered by warming and cooling. Changes in swelling are central to regeneraton and reuse.
Some separations using commercial gels are based on gel absorption, with resulting volume changes. These commercial gels absorb organic molecules. The gels used are hard to regenerate, and so are burned.
The separation closest to producing the results of the present invention is ultrafiltration. For that process, water and small molecular weight solutes are forced under high pressure through a size selective membrane. Larger molecular weight solutes are retained as concentrates. The membranes can be used for many separations and can be cleaned by reversing the flow of solvent. However, membrane cost is high and the application of pressure is expensive.